1. Field of the Invention
The present invention relates to a substrate within a Ni/Au structure electroplated on the electrical contact pads and a method for fabricating the same.
2. Description of the Prior Art
To catch the downsize trend of the electronics technology development, some issues have impacted the progress of substrates for integrated circuits (IC) packaged. On the surface of the substrate, electrical contact pads connected to a plurality of conducting wires is formed for the transmission of electronic signals or power. On the top surface of the electrical contact pads, a metal Ni/Au layer is formed. Some electrical contact pads, also known as bonding pads, of a substrate are covered with a Ni/Au layer on the surface. During the wire-bonding process, the metal wires and the bonding pads are all made of gold, resulting in excellent electrical coupling. Furthermore, other electrical contact pads of a substrate, solder ball pads for example, are covered with a Ni/Au layer electroplated on the surface, so that the conducting pads (usually made of copper composition) of the solder ball pads can be prevented from oxidation to improve the electrical interconnection performance of the solder ball pads.
Illustrated as FIG. 1, wherein a schematic diagram showing a Ni/Au structure electroplated on electrical contact pads of a substrate in the prior art. On the substrate 1, there are formed an upper circuit layout pattern 11, a lower circuit layout pattern 12, a plurality of plated through holes 13 or blind vias. Photolithography and etching are employed to define a circuit layout pattern with a plurality of electrical contact pads 10 (such as bonding pads or solder ball pads). Furthermore, there is a solder mask 14 to prevent soldering on the top surface of the substrate 1.
Even though the electrical contact pads 10 on the substrate 1 is disclosed to have a Ni/Au structure 16 electroplated. However, to obtain such a structure, it is required to dispose a plurality of conducting wires 15 for electroplating. Those conducting wires 15 are used to assist the electroplating process for forming the Ni/Au structure 16 electroplated on the electrical contact pads 10. Accordingly, a Ni/Au structure 16 is electroplated on the electrical contact pads 10, however a large amount of area will be occupied by the plurality of conducting wires, leading to reduce area for circuit layout pattern. In addition, noise due to the antenna effect when the conducting wires are employed may occur at high frequency. Even though etch-back can be used to remove the conducting wires 15, some residual may be left. Therefore, some residual are left leading to the antenna effect when a Ni/Au structure electroplated on the electrical contact pads of the substrate. Unfortunately, It still exists some defects in this issue such as reduced circuit layout area and noise generated at high frequency.
To overcome the problems stated above, the other method has been provided to improve the prior art, which is also referred to as Gold Pattern Plating (GPP). Illustrated as FIG. 2A to FIG. 2D, which shows the GPP process for electroplating a Ni/Au structure.
First, an electrically conducting layer 21 is formed on each surface of a substrate 2, illustrated as FIG. 2A, where a plurality of contact holes or blind vias will be formed.
A photoresist layer 22 is deposited on each of the electrically conducting layer 21 on the substrate 2, where there are a plurality of openings in the photoresist layer 22 to expose some regions, wherein the Ni/Au layer 23 is formed by electroplating method, illustrated as FIG. 2B.
Thereafter, the photoresist layers 22 are removed and the Ni/Au layer 23 still exists on the conducting layer 21, illustrated as FIG. 2C.
Finally, the Ni/Au layer 23 is used as a mask to define the electrical conducting layer 21 to form a circuit layout pattern 25 by photolithography and etching technology, wherein a Ni/Au layer 23 electroplated on the circuit layout pattern, illustrated as FIG. 2D. The other description drawing of the product illustrated as FIG. 2E.
Even though the conducting wires for electroplating the Ni/Au layer are not required in this case, the circuit layout pattern 25 (including electrical contact pads 26 and the electrically conducting traces) on each surface of the substrate 2 are coated with a Ni/Au layer, resulting in increasing fabrication cost since metal Ni/Au is very expensive. Furthermore, corresponding to the electrical contact pads 26 on the circuit layout pattern 25 on each surface of the substrate 2, only the top surface are covered with the Ni/Au 23, while the rest of the electrical contact pads 26 (including the side walls of the electrical contact pads serving as the bonding pads) are exposed without Ni/Au coverage. The exposed portions are corrosion due to oxidation, and the adhesion of bonding pads will be weak for lack of coverage with gold. Therefore, during the bonding process, electrodes on the chip and bonding pads on the substrate are bonded together by gold wires (not shown), the electrical coupling between gold wires and the bonding pads may be negative affected. Furthermore, the Ni/Au 23 on the circuit layout pattern 25 covered by the solder mask 27 (Illustrated as FIG. 2E) to prevent soldering on the top surface of the substrate 2 does not have reliable coupling due to lower adhesion between different materials resulting in poor yield.
Therefore, the present invention is to provide a substrate within a Ni/Au structure electroplated on electrical contact pads and a method for fabricating the same. The present invention does not necessary have additional conducting wires for electroplating process, so that the area occupied by the conducting wires for electroplating is decreased and the circuit layout area is increased. On the other hand, in the present invention, the electroplated contact pads are covered by Ni/Au, which facilitates the wire bonding process for packaged and the electrical contact pads to have excellent electrical interconnection performance, so as to improve the electrical coupling performance by means of reducing corrosion effect.